Cluster tools are highly integrated multiple-chamber semiconductor wafer processing systems adapted to perform a sequence of individual processing steps in the fabrication of device structures on substrates, such as semiconductor wafers. Cluster tools have various advantages over stand-alone tools. For example, cluster tools increase product yield and process throughput, reduce contamination, and require less human intervention. In particular, multiple processing steps can be accomplished in the cluster tool without exposing the substrates to surrounding ambient atmosphere between consecutive processing steps.
Cluster tools typically include multiple process modules each adapted to perform a process step and a material handling system or wafer handler. Traditionally, cluster tools comprise either multiple physical vapor deposition (PVD) modules arranged around a central wafer handler or multiple chemical vapor deposition (CVD) modules arranged around a central wafer handler. The central wafer handler includes a wafer transfer robot operable to transfer wafers to and from wafer cassettes and among the various process modules in a series of pick-and-place operations. In a common arrangement, the wafer handler and a group of wafers to be processed are housed within a transfer vacuum chamber maintained at a given vacuum pressure. Except during wafer exchanges, the process chamber of each process module is isolated from the transfer vacuum chamber by a gate or slot valve. An opening provided in each slot valve is dimensioned to pass the wafer and an end effector of the wafer transfer robot carrying the wafer.
The integration of multiple process chambers into a single platform increases the process throughput of the cluster tool. However, the arrival of copper metallization in device fabrication has introduced previously unrecognized concerns in the design of cluster tools. To optimize the throughput, copper interconnect technologies require the integration of CVD process modules and non-CVD process modules, such as PVD process modules, into a single cluster tool for the production of various metallization layers. For example, copper interconnect technologies that rely on the Damascene and the dual Damascene processes typically include the sequential process steps of a soft-etch cleaning, CVD of a barrier layer, for example Ta/TaN, and PVD of a copper seed layer. The bundling of CVD and non-CVD process modules, such as PVD process modules, into a single cluster tool improves system performance by reducing the total time needed to process groups of wafers, which increases the tool capacity.
The central wafer handler of such bundled cluster tools must service both CVD and non-CVD process modules. One problem that arises in such bundled systems is the migration of the gaseous reaction byproducts of the CVD process, including non-reacted surplus source gases, from the CVD process chamber into the transfer vacuum chamber, housing the central wafer handler, during wafer exchanges. When the slot valve isolating the CVD process chamber is opened to permit a wafer exchange, CVD reaction byproduct gases migrate or diffuse through the opening and escape into the transfer vacuum chamber. The CVD reaction byproduct gases persist as a contaminant in the transfer vacuum chamber. When the slot valve for one of the non-CVD process modules is opened for a wafer exchange, the CVD reaction byproduct gases in the transfer vacuum chamber can enter the non-CVD process chamber. In particular, CVD reaction byproduct gases can contaminate PVD process chambers and be unintentionally incorporated into the thin films being deposited by the PVD process chamber. The incorporation of the CVD reaction byproduct gases as an impurity can degrade the mechanical and electrical properties of the thin film. In particular, the CVD reaction byproduct gases from the CVD process that deposits the Ta/TaN barrier layer are particularly detrimental to the subsequent PVD process that deposits the copper seed layer.
Various solutions have been proposed for reducing or eliminating the migration of CVD reaction byproduct gases out of the CVD process chamber during wafer exchanges. One proposed solution is to evacuate the CVD process chamber to a base vacuum level before conducting a wafer exchange. However, significant amounts of residual CVD reaction byproduct gases remain in the chamber and can escape through the slot valve into the transfer vacuum chamber during wafer exchanges. Another proposed solution is to provide a flow of a purge gas into the transfer vacuum chamber before the wafer exchange to raise the vacuum pressure of the transfer vacuum chamber to a significantly higher value than the vacuum pressure in the CVD process chamber. When the slot valve for the CVD process chamber is opened, however, a burst of purge gas occurs into the CVD process chamber. The rapid flow of purge gas provides an unstable, quasi-turbulent condition that urges CVD reaction byproduct gases to exit into the transfer vacuum chamber.
With growing requirements for integrating CVD process modules and non-CVD process modules into a single cluster tool platform, the present invention provides apparatus and methods for significantly reducing or eliminating the migration of CVD reaction byproduct gases from the CVD process module into the transfer vacuum chamber during wafer exchanges so that non-CVD process modules, that share the transfer vacuum chamber with the CVD process module, will not be contaminated by the CVD reaction byproduct gases.